The invention relates to a method and a device for estimating the DC offset portion of a signal, especially of a signal containing parts with a sinusoidal shape, for example a signal which results from demodulation of a frequency modulated receive signal. After demodulation of a frequency modulated signal, the problem of carrier frequency estimation transforms into Direct Current (DC) estimation of the received signal. For simplicity only the abbreviated term DC estimation will be used in the present specification. The term tracking is used if the DC value is continuously estimated not only during the beginning of a packet but also for the remaining portion of a packet. In particular the present invention relates to a device for correcting the DC offset of a signal, for example a received radio frequency signal after demodulation.
The present invention is especially designed for systems where fast and reliable DC estimation is difficult due to the nature of the received signal, especially for received data packets in TDMA (Time division Multiple Access) and CDMA (Code Division Multiple Access) systems.
In some known systems of this kind (e.g. DECT, Digital European Cordless Telephone Standard), the DC estimation is not a problem due to a long sequence of equally distributed bits (1010101 . . . series) at the start of each data packet. The preamble is DC free and can be used for DC estimation. After the start of the packet is indicated by some control logic, the estimated DC is usually frozen and used for the rest of the packet. In the mentioned DECT standard, the preamble is long enough to apply linear methods of DC estimation, e.g. simple low pass filtering of the incoming signal by means of a R-C combination can be applied.
For systems having a shorter preamble, the DC estimation has to be faster which usually results in a lower accuracy of the estimated DC. One possibility to overcome the problem of the low accuracy of an initial estimate is not to freeze the rough DC estimate but to perform continuous tracking during the whole data packet. One difficulty with those DC tracking algorithms are sequences of equal bits that result in a signal that is basically constant. In such a case the estimated DC must not slowly drift to this constant signal. One example of a system with a short preamble is the Bluetooth™ system described in the specification of the Bluetooth™ standard. Those skilled in the art will appreciate that other wireless systems also exist, and reference herein to Bluetooth™ is not intended to be limited thereto.
Bluetooth™ wireless technology allows users to make effortless, wireless and almost instant connections between various communication devices, such as mobile phones, computers, printers, etc. Bluetooth™ provides short-range wireless connectivity and supports both point to point and point to multipoint access. The Bluetooth™ standard provides a preamble of data packets with a preamble of four bits which can be used for DC-estimation.
Bluetooth™ operates in the 2.4 GHz ISM band using a band of 83.5 MHz which locates 79 RF channels spaced 1 MHz apart. The channel is represented by a pseudo-random hopping sequence through available channels. The channel is divided into time slots where each slot corresponds to an RF hop frequency. Consecutive hops correspond to different RF hopping frequencies. The nominal hop rate is 1600 hops per second. All Bluetooth™ devices of a given piconet are time and frequency hop synchronized to the channel.
The foregoing description makes it apparent to those skilled in the art that fast and reliable DC estimation methods are necessary for operating a communication device which operates in accordance with the Bluetooth™ standard.